If she's growing at a normal rate (keeping in mind that for many babies, the explosive growth of the early months slows down between 4 and 6 months) then there is no reason to worry she's not getting enough to eat or that it's not nutritious. And it sounds like her weight gain has been great, so I wouldn't even worry about that.
This is normal behavior for an infant. All of mine nursed nearly constantly, in the first six to nine months. First of all, it's the optimal situation, biologically speaking. Frequent feeding is the normal pattern for human lactation. Second of all, nursing is about more than food. Yes, she's eating actively while she's nursing, so it's not "just" comfort nursing, but it is comfort nursing. The breast is home to them. They come to the breast because they're hungry, thirsty, lonely, tired, having trouble sleeping, overstimulated, cuddly, insecure, and for a thousand other reasons.
It can certainly wear you out, especially if you're working all day. But you're doing just exactly the right thing! And I think the suggestion that it's reverse cycling is probably correct--- the pump is not as good at maintaining your supply as she is, and all that frequent night nursing is going to keep you pumping plenty when you need it. And the way you see it reappear every few weeks-- she's alert to when your supply needs a boost, and doing what's needed to keep it up.
FWIW, though, I wouldn't hesitate to introduce a pacifier, to a baby four months old who has good weight gain. I would use it extremely cautiously, and keep a close eye that weight gain doesn't slow. I would use it not to replace feeds-- if she stops feeding frequently at night, you could see a decrease in your supply, if she's not with you all day. But it can help to get you a better stretch of night sleep, especially earlier in the night. I used it this way with my second daughter-- I'd nurse her until she was asleep, and then slipped my nipple out and slipped the paci in, and she'd sleep a bit longer before the next feed. It let me get a three hour stretch from about 9 to midnight, which was FANTASTIC.